Submenu layouts consist of a grid of menu items.
In order to use the submenu grid, your submenu needs to have a predetermined width. That means that you should either use a full width submenu or set a specific width in the Menu Item Submenu Settings. Learn more about Submenu Position and Submenu Width
Via the submenu settings, you can set a default grid layout for the submenu. For example, if you want a 6-column submenu, you can set the Submenu Columns Default to 1/6. The submenu items whose Columns Width settings is set to auto will automatically become a 1/6th grid column. Learn more about Submenu Columns
For more information on the UberMenu Grid, please see the UberMenu 3 Responsive Grid Demo
For a visual rundown of how submenu layouts work, check out the Submenu Layout Demo